If you used the Desktop software last year then you can transfer your information from your 2017 return into your 2018 return, that will also transfer your 2017 AGI. You can only transfer prior year information when you first start your 2018 return (before you enter any information). When you start your 2018 it will ask if you want to transfer your prior years information.
If you already started your return & don't want to start over, to open your .tax2017 file, you need the 2017 tax software installed on your computer. Or, if you saved your 2017 return as a PDF file, you can open the file in Adobe Reader to find your 2017 AGI.
Select your form to see where the AGI is located on each form.Form 1040 – Line 37